In a bowl, combine 2 tablespoons cornstarch, 2 tablespoons water and garlic powder until smooth. Add beef and toss.
Set a large skillet or wok over medium high heat, stir-fry beef in 1 Tbsp oil until beef reaches desired doneness; remove and keep warm. Stir-fry broccoli and onion in remaining oil for 4-5 minutes.
Return beef to pan. Combine soy sauce, brown sugar, ginger and remaining cornstarch and water until smooth; add to the pan. Cook and stir for 2 minutes. Serve over rice.